CNN  — 

Former national security adviser John Bolton’s surprise announcement Monday that he is willing to testify in the Senate’s impeachment trial significantly raises the already massive stakes of the pending votes in the chamber as to how the trial of President Donald Trump will be conducted. And it puts even more pressure on Senate Majority Leader Mitch McConnell as he seeks to keep his 52 GOP colleagues in line as both sides prepare for the historic proceeding.

At the core of the debate over how the Senate impeachment trial will work is whether or not witnesses will be allowed to be called. In mid-December, Senate Minority Leader Chuck Schumer sent a letter to McConnell requesting that four witnesses be allowed to testify – a list that included Bolton as well as acting White House chief of staff Mick Mulvaney.

McConnell promptly rejected that proposal from Schumer. “We don’t create impeachments,” McConnell said on the Senate floor. “We judge them.”

But here’s the catch: The rules of how a Senate trial will work – with regard to calling witnesses and all the rest – are determined by the Senate. In the last impeachment trial – for Bill Clinton in January 1999 – the rules package governing how the trial would proceed was approved 100-0. (Three witnesses were allowed to testify in that trial.) While McConnell has signaled that he will try to negotiate a deal with Schumer over the rules, it’s hard to see how the two will find common ground – especially over the question of witnesses.

All of which means that there will be a battle royale between the two leaders – as Schumer seeks to lure four Republican senators to support his push for witnesses while McConnell tries to keep a majority in support of holding the line.

Prior to Bolton’s announcement Monday, there were only small cracks in that Republican unity. Maine Sen. Susan Collins told a local radio station on New Year’s Eve that she was “open to witnesses,” before adding: “I think it’s premature to decide who should be called until we see the evidence that is presented and get the answers to the questions that we senators can submit through the chief justice to both sides.” Alaska Sen. Lisa Murkowski criticized McConnell for his coordination with the White House on impeachment but didn’t come out directly in support of calling witnesses. And Utah Sen. Mitt Romney, a frequent critic of Trump, was noncommittal last month when asked whether he supported the calling of witnesses. “It’s not that I don’t have any point of view; it’s just that I’m not willing to share that point of view till I’ve had the chance to talk to others and get their perspectives,” he told The Washington Post.

Bolton’s willingness to testify could very well change that math for McConnell. After all, Bolton, serving as national security adviser, was right in the heart of the administration’s action toward Ukraine – he was in that role when Trump spoke with Ukrainian President Volodymyr Zelensky in July 2019 and asked for the foreign leader to look into debunked allegations of wrongdoing by former Vice President Joe Biden and his son, Hunter. And it was Bolton who, according to former National Security Council staffer Fiona Hill, told her that he was “not part of whatever drug deal [US Ambassador to the European Union Gordon] Sondland and Mulvaney are cooking up.” (Hill testified to that under oath during the House impeachment hearings.)

In short: It’s not much of a leap to assume that Bolton could shed light on the question at the heart of the impeachment trial: Was Trump, in asking Zelensky for an investigation into a possible 2020 rival, using his office for personal and political gain? Or was he simply acting to protect American interests – and money – abroad?

What we don’t know is this: Does the possibility of Bolton testifying change the minds of Republican senators? (Remember that only four would need to side with Democrats in order for witnesses to be called.) Or can McConnell, who has proven over the past several decades to be one of the most able persuaders in American politics (witness Collins’ support for Supreme Court Justice Brett Kavanaugh), settle the undoubtedly jangled nerves within his conference in the wake of this Bolton stunner?

Either way, McConnell’s job got harder on Monday. Maybe much, much harder.